In August 2002 extreme floods affected Bohemia a part of the Czech Republic, along Vltava and Labe rivers. After the flood had culminated, soil samples of arable and meadow soil (flooded and control-non-flooded) have been collected in the surroundings of the cities of Ceske Budejovice (CB, South Bohemia, the Vltava river, the upper flood stream) and Usti nad Labem (UL, the Labe river, the lower flood stream). Different parameters of arable soils have been affected more significantly than those of meadow soils. Soil texture e.g. has been shifted from sandy loam to loam (CB) and clay loam (UL). Organic carbon, total phosphorus and nitrogen contents have been increased as well as the content of some hazardous elements such as arsenic. A shift of soil pH from 6.1 to 4.8 has been noticed in CB and an opposite one from 5.1 to 7.1 was measured in UL. Available phosphorus decreased in CB by 56% while a four-fold increase was measured in UL. Chlorinated organic pesticides level was significantly reduced, but that of polycyclic aromatic hydrocarbons (PAH) increased up to the maximum values of legal limits. The UL arable soil also became heavily contaminated by PAH. The CB arable soil demonstrated high microbial activity, while UL arable soil suffered under stress conditions in this respect. The flooded meadow soils became covered with a layer of drifted soil sediment, which retained enhanced content of the PAH’s different hazardous elements, and, in addition, the soil aeration was reduced

Laboratory experiments were conducted to study the effects of soil aeration on a soil microbial community in a cambisol with a pH of 5.3 and a carbon content of 2.2%. Anaerobically incubated soil samples (7 and 14 days) which were transferred to aerobic conditions showed a rapid decline (by 30-58%) and subsequent increase in microbial biomass. The decline of microbial biomass was accompanied by an increase in specific respiration rate of microorganisms and was reduced, but not eliminated, by the addition of glucose. Carbon extraction by 0.5 M K2SO4 (C-K2SO4) was used as a measure of the release of intracellular compounds from cells damaged due to aeration. C-K2SO4 did not decrease in the soil preincubated in anaerobic conditions for 7 days and no significant changes in direct counts of bacteria were found. Thus the observed decrease in microbial biomass can be explained more likely by shrinkage than by the damage of microbial cells. When the soil was preincubated in anaerobic conditions for 14 days, the decline in microbial biomass was accompanied by an increase in C-K2SO4 indicating death of a part of the microbial community. On the other hand, a rapid decline of hot-water extractable carbohydrates which had accumulated during anaerobic conditions occurred after aeration, which indicates acceleration of substrate utilization due to oxygen supply. The changes caused by aeration indicate the existence of a transient period with depressed microbial biomass but activated metabolism before a new equilibrium was reached.